Vancomycin MIC creep in non-vancomycin-intermediate Staphylococcus aureus (VISA), vancomycin-susceptible clinical methicillin-resistant S-aureus (MRSA) blood isolates from 2001-05

Objectives: To assess whether methicillin-resistant Staphylococcus aureus ( MRSA) vancomycin MIC shifts ( MIC creep) at a tertiary care institution occurred that may have gone undetected using traditional susceptibility markers ( percentage susceptible, MIC50, MIC90) over a 5 year period. Additionally, MIC trends were evaluated for oxacillin, linezolid and daptomycin. Methods: Etest MICs were performed on MRSA blood culture isolates ( January 2001 - December 2005). Only one isolate per patient was studied. The reported Etest MIC result was used and not rounded upward. MIC50, MIC90, median and geometric mean MIC, percentage susceptible and percentage resistant were calculated for each drug in each year. Non-parametric methods ( linear correlation and Mantel - Haenszel chi(2)) were used to assess MIC trends over time and the association of vancomycin, linezolid and daptomycin MICs with oxacillin MICs. Results: All isolates were susceptible to vancomycin, linezolid and daptomycin and resistant to oxacillin. MICs increased for vancomycin, linezolid and oxacillin ( P < 0.0001); however, daptomycin MICs decreased slightly ( P = 0.0386). For vancomycin, linezolid and oxacillin, there were significant increases ( P < 0.0001) in the percentage of isolates with MICs that were higher than the respective 2001 median MIC, but not for daptomycin ( P = 0.1361). Oxacillin MICs were associated with MICs of linezolid ( r = 0.364, P < 0.0001), vancomycin ( r = 0.353, P < 0.0001) and daptomycin ( r = 0.106, P = 0.0063). Conclusions: Oxacillin, vancomycin and linezolid MICs increased over time. For vancomycin and linezolid, these MIC increases were not reliably detected by percentage susceptibility as they occurred below the susceptibility breakpoint. Although the MICs of all agents appeared to be associated with increasing oxacillin MICs, the strongest associations were noted for vancomycin and linezolid.